The present invention relates to a melting device for melting a unit of ink for use in an inkjet printer, comprising a melting chamber provided with a wide end for dispensing the unit of ink to the melting chamber and to a narrow end thereof. The melting chamber has a form such that the ink unit moves, as a result of melting, in a direction from the wide end to the narrow end, the ink unit being laterally enclosed with respect to this direction by one or more walls of the melting chamber. The present invention also relates to an inkjet printer provided with a melting device of this kind.
This melting device is known from U.S. Pat. No. 5,030,972. The melting device is used to provide the inkjet printer printhead, which uses hot-melt melt ink, with liquid ink. Hot-melt ink, also known as phase-change ink, is an ink which is solid under normal ambient conditions but liquid at an elevated temperature. To enable this ink to be transferred to a receiving material by means of the inkjet printer printhead, the ink must be made liquid. During printing, the liquid ink is ejected in the form of individual droplets by the printhead in the direction of the receiving material. In this way, an image built up of a number of separate dots is formed on the receiving material. In order to melt the solid ink as quickly as possible without the need for considerable power, a solid unit of ink in the known device is brought into direct contact with a heater, which also keeps the ink liquid in the printhead. To achieve this, the melting chamber is formed into a constriction by a first vertical wall which acts as a heater (hereinafter referred to as the melting wall) and a second wall, and at an angle thereto which serves to keep the ink unit in contact with the melting wall. When an ink unit is dispensed into the melting chamber via the wide end of the constriction it is enclosed by the two walls. The ink unit melts there where it is in contact with the vertical melting wall, which is heated to above the temperature at which the ink is a liquid. As a result of this melting, the dimensions of the ink unit decrease, so that after the melted ink has been discharged it moves in the direction of the narrow end. In these conditions the melted ink is discharged through the small perforations in the melting wall. The transport of the ink through these perforations takes place by capillary forces.
A melting device of this kind has a significant disadvantage. Since the passage of melted ink to the inkjet head is dependent on the capillary action of the perforations in the vertical melting wall, the maximum speed at which melted ink can be supplied to the printhead is relatively low. This gives rise to problems, particularly if there is a considerable demand for liquid ink, for example if the inkjet printer has to print an illustration with a high degree of coverage, particularly a colored poster. An inadequate supply of melted ink can lead to the liquid ink in the printhead becoming exhausted, so that printing must be temporarily interrupted, which is a disadvantage with respect to inkjet printer productivity. Another problem that may arise in this connection is the inclusion of air bubbles in the liquid ink, which has a very adverse effect on the printing behavior of the printhead. An additional disadvantage of the low speed at which liquid ink is fed through the perforations is that a thin layer of liquid ink will form between the vertical melting wall and the solid ink unit. A layer of liquid ink of this kind forms a thermal barrier, so that the ink will melt more slowly under otherwise identical conditions. It is also a disadvantage with respect to the supply of melted ink to the printhead.
The object of the melting device according to the present invention is to obviate these disadvantages. To this end, a melting device is invented in which each of the one or more walls of the melting chamber is heated, during the melting process, to above the temperature at which the ink is liquid. It has surprisingly been found that in this way the supply of melted ink increases very considerably. In a melting device according to the invention in which the solid ink unit is heated to above its melting point at all places where it is enclosed by the one or more walls of the melting chamber, extra driving forces are apparently brought into play which cause the melted ink to be discharged at an accelerated rate. The reason for this highly accelerated supply is not completely clear, but closer research has indicated a number of possible causes. Firstly, an ink unit melted from a number of sides will decrease in its structure more rapidly, so that the unit can move more rapidly in the direction of the narrow end of the melting chamber. In the melting device according to the present invention, this movement is made possible by the fact that the unit of solid ink is enclosed only laterally with respect to the direction of movement, i.e. there is no support in a plane perpendicular to the direction of movement which would obstruct the movement of the ink unit. The movement results in a driving force which presses the already melted ink out of the contact surface between the solid ink unit and the melting wall. In addition, as a result of the more rapid movement, the ink unit will come into contact with a xe2x80x9cfreshxe2x80x9d melting wall surface at an earlier time. Since the thermal conductivity coefficient of the melting wall has a finite value, this means that the fresh melting wall surface has a higher temperature than the melting wall surface which has previously already given up its heat. Here again, the supply of melted ink can be further increased. An additional advantage of this is that the value of the thermal conductivity coefficient of the melting walls is less critical. In addition to the above-mentioned effects which already reinforce one another, it has been found that when an ink unit is laterally enclosed by one or more walls of the melting chamber, the solid ink unit is frequently pressed into contact on at least a part of the melting wall surface with a force greater than the force of gravity acting on the ink unit. A more powerful contact pressure force of this kind ensures that the melted ink is pressed more rapidly out of the contact surface, and this means an extra driving force for the transport of the already melted ink together with a reduction of the thermal barrier between the solid ink unit and the melting wall. These effects appear to reinforce one another in such a manner that a considerable supply of melted ink can be obtained. In addition to these effects, there are probably other causes whereby the supply of melted ink in a melting device according to the invention is considerable. However, knowledge of this is of secondary importance to the successful application of the invention.
In a preferred embodiment, the solid ink unit moves in the melting chamber by the force of gravity. This can be achieved by so disposing the melting chamber with respect to the gravity field that a nett force forms on the solid ink unit in the direction of the narrow end of the constriction. In this way no additional means are required to move the ink unit in the direction of the narrow end. Such means, for example in the form of a spring, not only increase the cost price of the melting device but also have the disadvantage that the dispensing of a following solid ink unit is rendered difficult. In the example given, the spring will have to be pressed in to place a new ink unit between the one or more melting walls of the melting chamber and the spring itself.
In a further, preferred embodiment, the melting chamber comprises, in the vicinity of the narrow end, at least one passage opening for the passage of melted ink. As a result of the provision of the passage opening, the melted ink which will move towards the narrow end under the influence of the force of gravity can leave the melting chamber. Transport of the melted ink will then take place so that it will finally reach the printhead.
In a further preferred embodiment, the melting chamber has an apex angle of less than 60xc2x0. This means that in the cross-section of the melting chamber parallel to the direction in which the solid ink unit is moved during melting there will be at least one place where the walls are at an angle of less than 60xc2x0. In this embodiment the melting chamber has the advantage that the contact pressure force acting on the solid ink unit perpendicularly to the one or more melting walls is greater than the force of gravity acting on the ink unit. As a result of this increased contact pressure force, melted ink is removed from the contact surface between the one or more melting walls and the solid ink unit at a faster rate. In addition, in a melting device of this kind, the contact surface between the solid ink unit and the one or more melting walls is increased, and this makes possible a further increase in the supply of melted ink. The combination of the two effects ensures that the speed at which a solid ink unit is melted increases greatly in a melting device according to this preferred embodiment.
In yet another preferred embodiment, the melting chamber has an apex angle of less than or equal to 40xc2x0. The contact pressure force and the contact surface is thus increased further. In addition, in a melting chamber of this kind, which is constricted relatively slowly, the distance covered by a melting ink unit is relatively considerable during melting. This means that room is readily made available in the melting chamber for a new solid ink unit to be dispensed, so that the supply of melted ink can be further increased.
In yet another preferred embodiment, the apex angle is greater than or equal to 5xc2x0 and less than or equal to 25xc2x0. When the apex angle is greater than or equal to 5xc2x0, the melting chamber can be made smaller for given dimensions of the solid ink unit, i.e. the distance between the wide end and the narrow end does not have to be as long. For a given length of the melting chamber, an apex angle greater than or equal to 5xc2x0 means that ink units of larger dimensions can be dispensed in the chamber. Utilizing an apex angle of less than or equal to 25xc2x0, a solid ink unit will move during melting more rapidly in the melting chamber.
In yet another preferred embodiment, the melting chamber has an apex angle greater than or equal to 12xc2x0 and less than or equal to 17xc2x0. This gives an optimum melting chamber in which the solid ink unit can be rapidly melted and an ink unit of sufficiently large dimensions can be dispensed in the chamber without the chamber requiring excessively large dimensions.
In one preferred embodiment, the melting chamber has means for discharging melted ink to the passage opening. This has two advantages. Firstly, the melted ink is then removed from the contact surface even faster, so that the thermal barrier between the melting walls and the solid ink unit becomes even smaller. On the other hand, this prevents the melted ink, which has a density less than that of the solid ink unit, from being pressed in the direction of the wide end. The consequence of this, in fact, would be that the solid ink unit would act as a plug in the melting chamber, with a quantity of melted ink collecting above the plug. This would not only temporarily interrupt the flow of melted ink to the passage opening and hence also to the inkjet head, but in addition the melting of the last solid ink of the plug might result in a sudden large supply of melted ink at the passage opening. A discontinuous supply of melted ink in this way is difficult to control. By the provision of the discharge means according to the invention, each recess formed in the wall surface of the one or more melting walls, particularly perforations, ribs, slots, grooves, ducts or a certain roughening, there forms in the melting chamber a continuous flow of melted ink to the passage opening. In a further preferred embodiment the discharge means comprise a slot formed in the one or more melting walls of the melting chamber. The slot has the advantage that it can be made so deep that a melting ink unit cannot practically assume a form such that the slot is completely filled and hence blocked by, as yet, unmelted ink. If the slot is formed helically, so that it does not extend parallel to the direction of movement of the melting solid ink unit, any blockage of the slot as a discharge means for the melted ink is further prevented. In a preferred embodiment, the melting chamber is substantially conical. Such a shape ensures that the ink unit for melting is enclosed on all sides so that the melting speed further increases. In addition, a form of this kind can be obtained simply by means of an injection moulding process.